Method for producing precipitate and catalyst

ABSTRACT

A molybdenum-based precipitate is prepared according to a process including the first step of forming a crude precipitate by pH adjustment to 6.5 or less in the presence of an alkali metal compound, and the second step of dissolving the crude precipitate in aqueous ammonia and forming a precipitate by pH adjustment to 6.5 or less. Then, the resulting molybdenum-based precipitate is washed with an acid aqueous solution having a pH of 6.5 or less and containing not less than 0.01 mole/L of ammonium root. Thus, a change in average particle diameter can be suppressed and good workability can be achieved, so that a molybdenum-based precipitate having a high purity and a desired average particle diameter can be obtained.

TECHNICAL FIELD

[0001] This invention relates to a process for the preparation ofprecipitates containing at least molybdenum and an element A (in which Ais at least one element selected from phosphorus and arsenic)(hereinafter also referred to as “molybdenum-based precipitates”). Moreparticularly, it relates to a process for the preparation ofmolybdenum-based catalysts useful for various reactions.

BACKGROUND ART

[0002] Molybdenum-based precipitates are useful as raw materials formolybdenum-based solid catalysts used, for example, in the production ofmethyl tert-butyl ether by the etherification of isobutene and methanol,the production of methacrylic acid by the dehydrogenation of isobutyricacid, and the production of methacrylic acid by the vapor phasecatalytic oxidation of methacrolein, and active investigations are beingcarried on for purposes of industrial production. In the prior art, alarge number of propositions have been made as to the compositions ofsuch catalysts and the preparation processes thereof. Generally, it isknown that a molybdenum-based precipitate can be formed by allowing anoxoacid ion containing the element A to exist in an aqueous solution ofa molybdate and acidifying the solution.

[0003] However, Conventional processes for the preparation of amolybdenum-based precipitate may fail to yield a molybdenum-basedprecipitate having a sufficiently high purity and a desired averageparticle diameter.

[0004] Moreover, most of the conventional processes for the preparationof a molybdenum-based precipitate are concerned with the preparation ofa molybdenum-based precipitate containing one or two metallic elements.Few reports have been made of a process which can also be suitablyapplied to the preparation of a molybdenum-based precipitate containingthree or more metallic elements.

[0005] On the other hand, a number of propositions have conventionallybeen made as to the method of washing a molybdenum-based precipitate.However, conventional methods for washing a molybdenum-based precipitateare disadvantageous in that impurities cannot be fully removed or aconsiderable change in average particle diameter may occurs duringwashing.

[0006] In particular, the average particle diameter of amolybdenum-based solid catalyst is an important factor which determinesthe pore distribution and other properties of the solid catalyst andhence contributes greatly to its performance such as catalytic activityand selectivity. Nevertheless, there have been few reports whichdisclose a method for washing a molybdenum-based precipitate efficientlywhile suppressing a change in the average particle diameter thereof.

[0007] Moreover, most of the conventional method for washing amolybdenum-based precipitate are concerned with the washing of amolybdenum-based precipitate containing one or two metallic elements.Few reports have been made of a method which can also be suitablyapplied to the washing of a molybdenum-based precipitate containingthree or more metallic elements.

[0008] In view of the above-described circumstances, an object of thepresent invention is to provide a process which permits amolybdenum-based precipitate having a high purity and a desired averageparticle diameter to be conveniently prepared with good workability, andwhich can be suitably applied to the washing of a molybdenum-basedprecipitate containing three or more metallic elements.

[0009] Another object of the present invention is to suppress a changein the average particle diameter of a molybdenum-based precipitate, washit conveniently with good workability, and thereby obtain amolybdenum-based precipitate having a high purity and a desired averageparticle diameter. A further object of the present invention is to washa molybdenum-based precipitate containing three or more metallicelements in a suitable manner.

DISCLOSURE OF THE INVENTION

[0010] In order to accomplish the above objects, the present inventionprovides a process for the preparation of a precipitate containing atleast molybdenum and an element A (in which A is at least one element ofphosphorus and arsenic), the process including:

[0011] the first step of adjusting the pH of a fluid mixture containingthe constituents of the precipitate to 6.5 or less in the presence of analkali metal compound, and filtering the crude precipitate so formed;and

[0012] the second step of dissolving the crude precipitate in aqueousammonia, adjusting the pH of the resulting solution to 6.5 or less, andfiltering the precipitate so formed.

[0013] It is preferable that, subsequently to at least one of the firststep and the second step, at least one of the crude precipitate obtainedin the first step and the precipitate obtained in the second step iswashed with an acid aqueous solution having a pH of 6.5 or less andcontaining not less than 0.01 mole/L of ammonium root.

[0014] Moreover, the present invention provides a process for thepreparation of a precipitate which includes the step of washing aprecipitate containing at least molybdenum and an element A (in which Ais at least one element of phosphorus and arsenic) with an acid aqueoussolution, wherein the acid aqueous solution has a pH of 6.5 or less andcontains not less than 0.01 mole/L of ammonium root.

[0015] Furthermore, the present invention provides a process for thepreparation of a molybdenum-based catalyst represented by the followinggeneral formula (1):

A_(a)Mo_(b)V_(c)Cu_(d)D_(e)Y_(f)Z_(g)O_(h)   (1)

[0016] wherein A is at least one element of phosphorus and arsenic, D isat least one element selected from the group consisting of antimony,bismuth, germanium, zirconium, tellurium, silver, selenium, silicon,tungsten and boron, Y is at least one element selected from the groupconsisting of iron, zinc, chromium, magnesium, tantalum, manganese,cobalt, barium, gallium, cerium and lanthanum, Z is at least one elementselected from the group consisting of potassium, rubidium and cesium, ais in the range of 0.5 to 3, b is equal to 12, c is in the range of 0 to3, d is in the range of 0 to 3, e is in the range of 0 to 3, f is in therange of 0 to 3, and h is the atomic ratio of oxygen required to meetthe valence of each element, the process including:

[0017] the first step of adjusting the pH of a fluid mixture containingat least molybdenum and the element A to 6.5 or less in the presence ofan alkali metal compound, and filtering the crude precipitate so formed;

[0018] the second step of dissolving the crude precipitate in aqueousammonia, adjusting the pH of the resulting solution to 6.5 or less, andfiltering the precipitate so formed; and

[0019] the step of preparing the catalyst from the precipitate.

BEST MODE FOR CARRYING OUT THE INVENTION

[0020] Preferred embodiments of the present invention will be describedhereinbelow.

[0021] The preparation process of the present invention includes atleast a first and a second step. In the first step, a fluid mixture isprepared by dispersing a molybdenum-based in water and adding an alkalimetal compound thereto. Thereafter, the pH of the resulting fluidmixture is adjusted to 6.5 or less, and the crude precipitate so formedis collected by filtration. In the second step, a solution is preparedby adding aqueous ammonia to the crude precipitate obtained in the firststep and dissolving the crude precipitate therein. Thereafter, the pH ofthe resulting solution is adjusted to 6.5 or less, and the precipitateso formed is collected by filtration.

[0022] As a result of intensive investigations on the preparation of amolybdenum-based precipitate, the present inventors have found that theabove-described process permits a molybdenum-based precipitate having ahigh purity and a desired average particle diameter to be convenientlyprepared with good workability and, moreover, the above-describedprocess can be suitably applied to the preparation of a molybdenum-basedprecipitate containing three or more metallic elements.

[0023] Although the reason therefor is not clearly known, it is presumedthat, by pH adjustment to 6.5 or less in the presence of ammonium root,the desirable precipitation of a molybdenum-based compound can beachieved in a state which minimizes the embedment of impurities and thelike.

[0024] No particular limitation is placed on the type of themolybdenum-based precipitate to which the present invention isapplicable, so long as the precipitate contains at least molybdenum andan element A (in which A is at least one element selected fromphosphorus and arsenic). Examples thereof include salts formed by thecombination of a heteropolyacid having molybdenum as the coordinatingelement and the element A as the central element, with an element Z (inwhich Z is at least one element selected from the group consisting ofpotassium, rubidium and cesium), and mixtures containing such acids.

[0025] As the basic structures of heteropolyacids, the Keggin andSilverton structures in which the ratio of the central element to thecoordinating element is 1:12, the Dawson structure in which the ratio is2:18, the Anderson structure in which the ratio is 1:6, and the like areknown. Such heteropolyacids may be prepared, for example, by acidifyingan aqueous solution containing a molybdate and an oxoacid ion containingthe element A so as to precipitate a heteropolyacid. In order to preparea heteropolyacid containing the element Z, a Z salt of theheteropolyacid may readily be formed by precipitating the heteropolyacidin the presence of a Z ion produced by dissolving a compound of theelement Z.

[0026] Since the molybdenum-based precipitate obtained by thepreparation process of the present invention has a high purity and adesired average particle diameter, a molybdenum-based solid catalyst(also referred to as “molybdenum-based catalyst”) having goodcharacteristics may be prepared as required, for example, by adding oneor more metallic elements to the molybdenum-based precipitate obtainedin the present invention.

[0027] That is, a molybdenum-based catalyst may be prepared by preparinga molybdenum-based precipitate having a desired average particlediameter according to a process including the aforesaid first and secondsteps, and subjecting the molybdenum-based precipitate to a subsequentstep for enhancing its catalytic performance (for example, addingthereto one or more metallic elements depending on the reaction system).

[0028] Examples of the molybdenum-based solid catalyst includemolybdenum-based catalysts represented by the following general formula(1):

A_(a)Mo_(b)V_(c)Cu_(d)Y_(f)Z_(g)O_(h)   (1)

[0029] wherein A is at least one element of phosphorus and arsenic, D isat least one element selected from the group consisting of antimony,bismuth, germanium, zirconium, tellurium, silver, selenium, silicon,tungsten and boron, Y is at least one element selected from the groupconsisting of iron, zinc, chromium, magnesium, tantalum, manganese,cobalt, barium, gallium, cerium and lanthanum, Z is at least one elementselected from the group consisting of potassium, rubidium and cesium, ais in the range of 0.5 to 3, b is equal to 12, c is in the range of 0 to3, d is in the range of 0 to 3, e is in the range of 0 to 3, f is in therange of 0 to 3, and h is the atomic ratio of oxygen required to meetthe valence of each element.

[0030] From the viewpoint of catalytic efficiency, handleability duringwashing, and the like, the average particle diameter of themolybdenum-based precipitate is preferably not less than 1 μm and morepreferably not less than 3 μm, and is preferably not greater than 100 μmand more preferably not greater than 70 μm.

[0031] Specific examples of the molybdenum-based catalyst include acatalyst for the production of methacrylic acid by the vapor phasecatalytic oxidation of methacrolein, a catalyst for the production ofmethyl tert-butyl ether by the etherification of isobutene and methanol,and a catalyst for the production of methacrylic acid by thedehydrogenation of isobutyric acid.

[0032] In the first step of the above-described process for thepreparation of a molybdenum-based catalyst, a molybdenum-based catalysthaving been used for any of various reactions (also referred to as “usedcatalyst”) may be used as the raw material for a molybdenum-basedprecipitate. In such a case, a used molybdenum-based catalyst whoseactivity has been reduced as a result of long-term use may be recovered,purified according to the process for the preparation of amolybdenum-based precipitate in accordance with the present invention,and then replenished with one or more necessary metallic elements. Thus,it is possible to prepare a reactivated molybdenum-based catalyst or amolybdenum-based catalyst having different reaction characteristics andother characteristics as compared with that before being recovered.

[0033] The first step of the present invention is usually carried out bydispersing a solid containing the constituents of a molybdenum-basedprecipitate in water and then adding an alkali metal compound thereto.Although no particular limitation is placed on the amount of wateradded, it should be used in such an amount as to permit the resultingmixture to be handled as a slurry. Usually, water is used in an amountof not less than 1 part by mass per part by mass of the solid containingthe constituents. The amount of alkali metal compound added ispreferably such that the pH of the resulting mixture will not less than8, more preferably not less than 8.5, and not greater than 12. Noparticular limitation is placed on the type of the alkali metal compoundused for this purpose. However, examples thereof include sodiumhydroxide, potassium hydroxide, cesium hydroxide and sodium hydrogencarbonate. Among others, sodium hydroxide is preferred. The alkali metalcompound may be added to the mixture, in the form of a solid or apreformed aqueous solution.

[0034] After the addition of the alkali metal compound, the mixture ispreferably kept for a predetermined time in order to dissolve themetallic elements present in the solid containing the constituents ofthe molybdenum-based precipitate. The keeping time preferably rangesfrom 0.5 hour to 24 hours, and the keeping temperature preferably rangesfrom room temperature to 90° C. Although the mixture may be allowed toremain stationary, it is preferably kept with stirring.

[0035] Thereafter, if the mixture contains any insoluble residue, it ispreferably removed by filtration or other means.

[0036] Then, the pH of this mixture is adjusted to 6.5 or less by theaddition of an acid. The acids which can be added for the purpose of pHadjustment include, for example, hydrochloric acid, nitric acid andsulfuric acid. Among others, hydrochloric acid and nitric acid arepreferred.

[0037] After pH adjustment, the mixture is preferably kept for apredetermined time in order to form a crude precipitate. The keepingtime preferably ranges from 0.5 hour to 24 hours, and the keepingtemperature preferably ranges from room temperature to 90° C. Althoughthe mixture may be allowed to remain stationary, it is preferably keptwith stirring.

[0038] On the basis of compositional analysis and X-ray diffractometry,it is presumed that the principal component of the crude precipitatethus obtained comprises, for example, a so-called Dawson-typeheteropolyacid salt in which the ratio of the central element (e.g.,phosphorus) to molybdenum is 2:18, or a mixture of a so-calledKeggin-type heteropolyacid salt in which the ratio of the centralelement (e.g., phosphorus) to molybdenum is 1:12, and a Dawson-typeheteropolyacid. In this case, the proportion of the Keggin-typeheteropolyacid salt becomes higher as the adjusted pH is lowered.

[0039] Where the amount of the element Z is not sufficient toprecipitate the heteropolyacid as a salt of the element Z, it ispreferable to add a source of ammonium root prior to pH adjustment sothat ammonium root will preferably be present in an amount of not lessthan 0.5 mole, more preferably not less than 3 moles, and not greaterthan 40 moles, per mole of the element A. The presence of ammonium rootpermits a larger amount of the heteropolyacids to be precipitated as anammonium salt, so that the recovery of molybdenum and the element Acontained in the crude precipitate can be improved. As the amount ofammonium root increases, the recovery of molybdenum and the element Abecomes higher.

[0040] No particular limitation is placed on the type of the source ofammonium root, so long as it is soluble. Examples thereof includeaqueous ammonia, ammonium chloride, ammonium nitrate and ammoniumcarbonate.

[0041] The residual fluid from which the molybdenum-based precipitateformed in the second step has been separated contains a large amount ofammonium root. Although this residual fluid may be discarded, it can beused as the source of ammonium root which is added to the mixture of thefirst step. This provides an economically desirable process because noadditional fresh source of ammonium root is required.

[0042] Where the constituents of the precipitate include at least oneelement selected from the group consisting of potassium, rubidium andcesium, the at least one element selected from the group consisting ofpotassium, rubidium and cesium is substantially removed, if necessary,prior to the first step.

[0043] More specifically, depending on the intended use of the finallyobtained molybdenum-based precipitate, it may be desirable that theelement Z is present in small amounts or completely absent. In such acase, it is preferable to remove all or part of the element Z from themixture prior to its pH adjustment to 6.5 or less. This may beaccomplished, for example, by causing Z ions to become adsorbed to acation-exchange resin. The removal of the element Z is preferablycarried out prior to pH adjustment to 6.5 or less. In this case, it isespecially preferable to add ammonium root so as to recover molybdenumand the element A efficiently.

[0044] The crude precipitate formed by pH adjustment is separated fromthe residual fluid by a suitable solid-liquid separation technique. Noparticular limitation is placed on the solid-liquid separation techniqueemployed, and any common techniques such as filtration andcentrifugation may be employed. For this purpose, there may be used anyof various common apparatus such as pressure filters, vacuum filters,filter presses and centrifugal separators.

[0045] In the second step, the crude precipitate obtained in theabove-described manner is redissolved by adding aqueous ammonia thereto.Then, a precipitate is formed again by adding an acid so as to adjustthe pH of the solution to 6.5 or less. No particular limitation isplaced on the state of the crude precipitate to which aqueous ammonia isadded, and the crude precipitate may be either in a dry state or in awet state.

[0046] The amount of aqueous ammonia added may be such that it candissolve the crude precipitate. However, it is preferable to add aqueousammonia in such an amount as to give a pH of 8 or greater. Aqueousammonia may be added directly to the crude precipitate. Alternatively,it is possible to disperse the crude precipitate in water and then addaqueous ammonia thereto. The acid used for the purpose of pH adjustmentmay be the same as that used in the first step, or may be differenttherefrom. This acid may be selected according to the intended use ofthe molybdenum-based precipitate. After pH adjustment, the solution ispreferably kept for a period of time ranging from 0.5 hour to 24 hours.The keeping temperature preferably ranges from room temperature to 90°C. Moreover, the solution is preferably kept with stirring.

[0047] Similarly to the first step, the precipitate formed in the secondstep is separated from the residual fluid by a suitable solid-liquidseparation technique.

[0048] Subsequently to the first step, the crude precipitate obtained inthe first step is preferably washed with an acid aqueous solution havinga pH of 6.5 or less and containing not less than 0.01 mole/L of ammoniumroot.

[0049] Moreover, subsequently to the second step, the precipitateobtained in the second step is preferably washed with an acid aqueoussolution having a pH of 6.5 or less and containing not less than 0.01mole/L of ammonium root.

[0050] By employing this washing method, a change in the averageparticle diameter of the molybdenum-based precipitate during washing canbe suppressed, and the molybdenum-based precipitate can be convenientlywashed with good workability without causing any significant alterationin the solid-liquid separation properties of the washing fluid and theprecipitate. Thus, there can be obtained a molybdenum-based precipitatehaving a desired average particle diameter. Moreover, there can beobtained a molybdenum-based precipitate having a sufficiently highpurity. Furthermore, even a molybdenum-based precipitate containingthree or more metallic elements may also be conveniently be washed withgood workability.

[0051] In order to suppress a change in the average particle diameter ofthe molybdenum-based precipitate during washing to a full degree, theconcentration of ammonium root in the acid aqueous solution shouldpreferably be not less than 0.01 mole/L, more preferably not less than0.05 mole/L, and most preferably not less than 0.1 mole/L.

[0052] The impurities contained in the molybdenum-based crudeprecipitate and precipitate include, for example, excess ammonium rootand nitrate root, chlorine, sulfur, salts formed from the alkali metalcompound added and the acid used for pH adjustment. The components inquestion may differ according to the intended use of the precipitatecompound. Accordingly, it is preferable to select a washing fluidsuitably according to the intended use and solubility of theprecipitate. From this point of view, the washing fluid used in thepresent invention may comprise, for example, at least one aqueoussolution selected from the group consisting of an aqueous solution ofammonium nitrate, an aqueous solution of ammonium chloride, and anaqueous solution of ammonium sulfate.

[0053] No particular limitation is placed on the washing method. Usablewashing methods include, for example, dispersion washing in which thecrude precipitate or the precipitate is dispersed in a washing fluid andthis dispersion is then subjected to solid-liquid separation, andpassage washing in which a washing fluid is passed through the crudeprecipitate or the precipitate in the form of a cake. The washing may becarried out at a temperature ranging from 0° C. to 90° C. However, withconsideration for the solubility of the precipitate and other factors,it is preferable to employ a temperature ranging from room temperatureto 50° C. No particular limitation is placed on the state of the crudeprecipitate and the precipitate after washing, and they may be either ina wet state or in a dry state.

[0054] By employing the above-described washing method, the crudeprecipitate and the precipitate can be made highly pure without causinga substantial change in the average particle diameter thereof.

[0055] Specifically, the rate of change in the average particle diameterof the crude precipitate before and after washing can preferably bereduced to not greater than 50%, more preferably not greater than 40%,and most preferably not greater than 30%.

[0056] Moreover, the rate of change in the average particle diameter ofthe precipitate before and after washing can preferably be reduced tonot greater than 50%, more preferably not greater than 40%, and mostpreferably not greater than 30%.

[0057] The term “rate of change in average particle diameter” as usedherein means a value defined by 100×|r−r′|/r in which r is the averageparticle diameter before washing and r′ is the average particle diameterafter washing.

[0058] By employing the above-described washing method, the amount ofchlorine contained in the washed precipitate obtained by washing theprecipitate obtained in the second step can preferably be reduced to notgreater than 0.10 mole, more preferably not greater than 0.05 mole, andmost preferably not greater than 0.03 mole, per mole of the element A.

[0059] Moreover, the amount of sodium contained in the washedprecipitate obtained by washing the precipitate obtained in the secondstep can preferably be reduced to not greater than 0.10 mole, morepreferably not greater than 0.05 mole, and most preferably not greaterthan 0.03 mole, per mole of the element A.

[0060] One example of the molybdenum-based precipitate washed in theabove-described steps comprises a precipitate obtained by recovering amolybdenum-based catalyst having been used for any of various reactions(also referred to as “used catalyst”) and forming it into a precipitateaccording to a suitable procedure. In such a case, a usedmolybdenum-based catalyst whose activity has been reduced as a result oflong-term use may be recovered, formed into a precipitate, and washedaccording to the method of the present invention. Thereafter, it may bereplenished with one or more necessary metallic elements. Thus, it ispossible to prepare a reactivated molybdenum-based catalyst or amolybdenum-based catalyst having different reaction characteristics andother characteristics as compared with that before being recovered.

[0061] Another example of the molybdenum-based precipitate comprises aprecipitate obtained by preparing a solution containing at leastmolybdate ion, an oxoacid ion containing the element A, and othermetallic acid ions as required, and adjusting the pH of the solution to6.5 or less.

[0062] After a molybdenum-based precipitate having a desired averageparticle diameter is prepared according to the above-described washingmethod, a molybdenum-based catalyst may be prepared by subjecting themolybdenum-based precipitate to a subsequent step for enhancing itscatalytic activity (for example, adding thereto one or more metallicelements depending on the reaction system).

[0063] The above-described washing method is suitable for the purpose ofwashing the crude precipitate obtained in the first step and theprecipitate obtained in the second step. However, it is to be understoodthat its use is not limited thereto. That is, the above-describedwashing method can be applied to any precipitate containing at leastmolybdenum and the element A, irrespective of its preparation process.

[0064] The present invention is more fully explained with reference tothe following specific examples. However, these examples are not to beconstrued to limit the scope of the invention. As to the reagents,commercially available products of high purity were used unlessotherwise specified.

Analytical Methods

[0065] The quantitative analysis of constituent elements (or molecules)was carried out by ICP emission spectrometry (using CID High-frequencyPlasma Emission Spectroscopic Analyzer, IRIS Advantage IP, manufacturedby Nippon Jarrell-Ash Co., Ltd.), atomic absorption spectrometry (usingSAS7500, manufactured by Seiko Instruments Inc.), ion chromatography(using DX-AQ2211, manufactured by Nippon Dionex Co., Ltd.), and theKjeldahl method.

[0066] The average particle diameter of a precipitate was determined bymeasuring its particle size distribution with an SK LASER MICRON SIZERPRO-7000 (manufactured by Seishin Enterprise Co., Ltd.) and finding aparticle diameter at which the cumulative frequency was equal to 50%.

EXAMPLE 1

[0067] (a) Use of a Molybdenum-Based Catalyst

[0068] A tubular reactor was charged with a catalyst containing 34.54parts by mass of molybdenum, 0.93 part by mass of phosphorus, 1.41 partsby mass of potassium, 0.76 part by mass of vanadium, and 0.57 part bymass of copper and having the composition ofP₁Mo₁₂K_(1.2)V_(0.5)Cu_(0.3), exclusive of oxygen (the same shall applyhereinafter). Then, a gaseous mixture composed of 5% by volume ofmethacrolein, 10% by volume of oxygen, 30% by volume of water vapor, and55% by volume of nitrogen was reacted for 2,000 hours under reactionconditions including a reaction temperature of 270° C. and a contacttime of 3.6 seconds.

[0069] (b) Removal of Potassium

[0070] After completion of the reaction, the catalyst was removed fromthe tubular reactor, and 91 parts by mass of this used catalyst wasdispersed in 400 parts by mass of pure water. After 89.2 parts by massof a 45% aqueous solution of sodium hydroxide was added thereto and theresulting mixture was stirred for 2 hours, the residue was filtered off.The filtrate was freed of potassium by passing it through a stronglyacid Na-type styrene-based ion-exchange resin (Amberlite IR-120B,manufactured by Organo Corp.) at a space velocity (SV) of 1.

[0071] (c) First Step

[0072] After the pH of the effluent fluid was adjusted to 8.9 by theaddition of 29.9 parts by mass of 36% hydrochloric acid, 19.25 parts bymass of ammonium chloride (i.e., 12.0 moles of ammonium root per mole ofphosphorus) was added thereto. Then, 84.1 parts by mass of 36%hydrochloric acid was added thereto, with stirring, so as to adjust thepH of the mixture to 1.0. As the pH became lower by the addition ofhydrochloric acid, a crude precipitate was formed. When the pH of themixture reached 1.0, it was in the form of a slurry. Thereafter, theslurry was kept at room temperature for 2 hours with stirring. Theresulting slurry containing the crude precipitate was filtered with aNutsche type vacuum filter to obtain a wet crude precipitate.

[0073] (d) Washing Step

[0074] The wet crude precipitate thus obtained was dispersed in 2 partsby mass of a 2% aqueous solution of ammonium nitrate (having a pH of 5.2at room temperature and containing 0.25 mole/L of ammonium root) andkept in the dispersed state for 5 minutes. The dispersion was filteredagain with a Nutsche type vacuum filter to obtain 120.5 parts by mass ofa wet crude precipitate.

[0075] (e) Second Step

[0076] The wet crude precipitate thus obtained was added to 500 parts bymass of pure water. After 71.1 parts by mass of 25% aqueous ammonia wasadded thereto, the resulting mixture was kept for 10 minutes withstirring. Initially, the mixture was in the form of a slurry, but becamea perfect solution after 5 minutes. At that time, its pH was 8.6. Then,after the temperature of the solution was raised to 70° C., 99.8 partsby mass of 36% hydrochloric acid was added thereto so as to adjust thepH of the mixture to 5.0. This mixture was kept at 80° C. for 3 hourswith stirring to form a precipitate. After the slurry containing theprecipitate was cooled to room temperature, it was filtered to obtain awet precipitate.

[0077] (f) Washing Step

[0078] Next, the wet precipitate was dispersed in and washed with 2parts by mass of a 2% aqueous solution of ammonium nitrate (having a pHof 5.2 at room temperature and containing 0.25 mole/L of ammonium root).After this procedure was repeated twice, the dispersion was filtered.Finally, 110.5 parts by mass of a wet precipitate was obtained. This wetprecipitate contained 30.55 parts by mass (12 atm. %) of molybdenum,0.90 part by mass (1.1 atm. %) of phosphorus, and 0.61 part by mass(0.45 atm. %) of vanadium; 11.51 moles of ammonium root per mole ofphosphorus; and impurities such as 0.01 mole of sodium and 0.02 mole ofchlorine, per mole of phosphorus.

[0079] When the wet precipitate was dispersed in the filtrate from whichthe wet precipitate had been separated in “(e) Second step”, and usedfor the measurement of its average particle diameter, it was found to be11.5 μm.

[0080] From the above-described results, it has been found that, byemploying a process including the first and second step in accordancewith the present invention, a molybdenum-based precipitate containingthree or more metallic elements and having good characteristics can beprepared with good workability.

[0081] Moreover, it has been found that a molybdenum-based precipitatecan also be satisfactorily prepared when the potassium contained in theabove molybdenum-based precipitate is replaced by sodium.

EXAMPLE 2

[0082] 125.1 parts by mass of a wet precipitate was obtained in the samemanner as described in Example 1, (a) to (d). This wet precipitatecontained 32.54 parts by mass of molybdenum, 0.91 parts by mass ofphosphorus, and 0.66 part by mass of vanadium; 10.71 moles of ammoniumroot per mole of phosphorus; and impurities such as 0.21 mole of sodiumand 0.15 mole of chlorine, per mole of phosphorus.

[0083] Subsequently, without carrying out the second step, the wetprecipitate was dispersed in and washed with a 2% aqueous solution ofammonium nitrate. This procedure was repeated three times [a total offour times, including the step of (d)]. For the respective washingsteps, the amount of residual sodium per mole of phosphorus decreased to0.13 mole, 0.11 mole and 0.11 mole. However, no appreciable decrease inresidual sodium was noted in and after the third washing step.Similarly, the amount of residual chlorine per mole of phosphorusdecreased to 0.12 mole, 0.09 mole and 0.09 mole. However, no appreciabledecrease in residual chlorine was noted in and after the third washingstep.

EXAMPLE 3

[0084] (a) Use of a Molybdenum-Based Catalyst

[0085] A tubular reactor was charged with a catalyst containing 34.54parts by mass of molybdenum, 0.93 part by mass of phosphorus, 1.41 partsby mass of potassium, 0.76 part by mass of vanadium, 0.57 part by massof copper, and 1.12 parts by mass of arsenic, and having the compositionof P₁Mo₁₂V_(0.5)As_(0.5)Cu_(0.3)K₁₂. Then, a gaseous mixture composed of5% by volume of methacrolein, 10% by volume of oxygen, 30% by volume ofwater vapor, and 55% by volume of nitrogen was reacted for 2,000 hoursunder reaction conditions including a reaction temperature of 270° C.and a contact time of 3.6 seconds.

[0086] (b) First Step

[0087] After completion of the reaction, the catalyst was removed fromthe tubular reactor, and 94 parts by mass of this used catalyst wasdispersed in 400 parts by mass of pure water. After 89.0 parts by massof a 45% aqueous solution of sodium hydroxide was added thereto and theresulting mixture was stirred for 1 hour, the residue was filtered off.After 29.8 parts by mass of 36% hydrochloric acid was added thereto soas to adjust the pH of the solution to 9.0, 28.90 parts by mass (i.e.,12.0 moles of ammonium root per mole of the combined amount ofphosphorus and arsenic) of ammonium chloride was added thereto. Then,48.0 parts by mass of 36% hydrochloric acid was added thereto so as toadjust the pH of the mixture to 5.0, and this mixture was kept at 60° C.for 3 hours with stirring. After this mixture was cooled to roomtemperature, it was filtered with a Nutsche type vacuum filter to obtaina wet crude precipitate.

[0088] (c) Washing Step

[0089] The wet crude precipitate thus obtained was dispersed in 2 partsby mass of a 2% aqueous solution of ammonium nitrate (having a pH of 5.2at room temperature and containing 0.25 mole/L of ammonium root) andkept in the dispersed state for 5 minutes. The dispersion was filteredto obtain 90 parts by mass of a wet crude precipitate.

[0090] (d) Second Step

[0091] The wet crude precipitate thus obtained was added to 500 parts bymass of pure water. After 55.5 parts by mass of 25% aqueous ammonia wasadded thereto, the resulting mixture was kept for 10 minutes withstirring. At that time, its pH was 8.7. Then, 78.1 parts by mass of 36%hydrochloric acid was added thereto so as to adjust the pH of themixture to 5.0, and this mixture was kept at 60° C. for 3 hours withstirring. After this mixture was cooled to room temperature, it wasfiltered to obtain a wet precipitate.

[0092] (e) Washing Step

[0093] The wet precipitate thus obtained was dispersed in and washedwith 2 parts by mass of a 2% aqueous solution of ammonium nitrate(having a pH of 5.2 at room temperature and containing 0.25 mole/L ofammonium root). This procedure was repeated twice. Finally, 83 parts bymass of a wet precipitate was obtained. This wet precipitate contained24.11 parts by mass (12 atm. %) of molybdenum, 0.56 part by mass (0.86atm. %) of phosphorus, 0.56 part by mass (0.69 atm. %) of potassium,0.36 part by mass (0.34 atm. %) of vanadium, and 1.12 parts by mass(0.71 atm. %) of arsenic; 9.42 moles of ammonium root per mole ofphosphorus; and impurities such as 0.01 mole of sodium and 0.02 mole ofchlorine, per mole of the combined amount of phosphorus and arsenic.

[0094] When the wet precipitate was dispersed in the filtrate from whichthe wet precipitate had been separated in “(d) Second step”, and usedfor the measurement of its average particle diameter, it was found to be16.2 μm.

[0095] From the above-described results, it has been found that, byemploying a process including the first and second step in accordancewith the present invention, a molybdenum-based precipitate containingthree or more metallic elements and having good characteristics can beprepared with good workability.

EXAMPLE 4

[0096] (a) First Step

[0097] 94 parts by mass of a used catalyst having the same compositionas that of Example 3 was dispersed in 400 parts by mass of pure water.After 89.0 parts by mass of a 45% aqueous solution of sodium hydroxidewas added thereto and the resulting mixture was stirred for 1 hour, theresidue was filtered off. Subsequently, in place of-ammonium chloride,640 parts by mass of the filtrate obtained in the second step of Example3 (containing 11 moles of ammonium root per mole of the combined amountof phosphorus and arsenic contained in the solution prior to theaddition of the filtrate) was added thereto. Then, 51.2 parts by mass of36% hydrochloric acid was added thereto so as to adjust the pH of themixture to 5.0, and this mixture was kept at 60° C. for 3 hours withstirring. After this mixture was cooled to room temperature, it wasfiltered in the same manner as in Example 3 to obtain a wet crudeprecipitate.

[0098] (b) Washing Step

[0099] Next, the wet crude precipitate was dispersed in and washed with2 parts by mass of a 2% aqueous solution of ammonium nitrate (having apH of 5.2 at room temperature and containing 0.25 mole/L of ammoniumroot). Thus, there was obtained 95 parts by mass of a wet crudeprecipitate.

[0100] (c) Second Step

[0101] The wet crude precipitate thus obtained was added to 500 parts bymass of pure water. After 57.5 parts by mass of 25% aqueous ammonia wasadded thereto, the resulting mixture was kept for 10 minutes withstirring. Then, 78.1 parts by mass of 36% hydrochloric acid was addedthereto so as to adjust the pH of the mixture to 5.0, and this mixturewas kept at 60° C. for 3 hours with stirring. After this mixture wascooled to room temperature, it was filtered to obtain a wet precipitate.

[0102] (d) Washing Step

[0103] The wet precipitate thus obtained was dispersed in and washedwith 2 parts by mass of a 2% aqueous solution of ammonium nitrate(having a pH of 5.2 at room temperature and containing 0.25 mole/L ofammonium root). This procedure was repeated twice. Finally, 89.7 partsby mass of a wet precipitate was obtained. This wet precipitatecontained 25.34 parts by mass (12 atm. %) of molybdenum, 0.57 part bymass (0.84 atm. %) of phosphorus, 0.56 part by mass (0.65 atm. %) ofpotassium, 0.38 part by mass (0.34 atm. %) of vanadium, and 1.12 partsby mass (0.68 atm. %) of arsenic; 9.31 moles of ammonium root per moleof phosphorus; and impurities such as 0.006 mole of sodium and 0.02 moleof chlorine, per mole of the combined amount of phosphorus and arsenic.

[0104] When the wet precipitate was dispersed in the filtrate from whichthe wet precipitate had been separated in “(c) Second step”, and usedfor the measurement of its average particle diameter, it was found to be17.8 μm.

[0105] From the above-described results, it has been found that, byemploying a process including the first and second step in accordancewith the present invention, a molybdenum-based precipitate containingthree or more metallic elements and having good characteristics can beprepared with good workability.

[0106] It has also been found that the filtrate obtained in the secondstep can be used in the first step.

EXAMPLE 5

[0107] 95 parts by mass of a wet precipitate was obtained in the samemanner as described in Example 3, (a) to (c). This wet precipitatecontained 26.16 parts by mass of molybdenum, 0.57 part by mass ofphosphorus, 0.58 part by mass of potassium, 0.37 part by mass ofvanadium, and 1.12 parts by mass of arsenic; 9.51 moles of ammonium rootper mole of phosphorus; and impurities such as 1.20 moles of sodium and1.12 moles of chlorine, per mole of phosphorus.

[0108] Subsequently, without carrying out the second step, the wetprecipitate was dispersed in and washed with a 2% aqueous solution ofammonium nitrate. This procedure was repeated five times. When thefinally obtained wet precipitate was analyzed, the amounts of residualsodium and residual chlorine per mole of the combined amount ofphosphorus and arsenic were 0.06 mole and 0.05 mole, respectively.

EXAMPLE 6

[0109] (a) Preparation of a Molybdenum-Based Precipitate

[0110] 63.62 parts by mass of ammonium paramolybdate, 1.05 parts by massof ammonium metavanadate, and 7.61 parts by mass of cesium nitrate weredissolved in 300 parts by mass of pure water at 70° C. After a solutionprepared by dissolving 3.46 parts by mass of 85% phosphoric acid in 10parts by mass of pure water was added thereto, 46.1 parts by mass of

[0111]36% hydrochloric acid was added thereto so as to adjust the pH ofthe mixture to 2.5, and its temperature was raised to 95° C. withstirring. Then, a solution prepared by dissolving 1.45 parts by mass ofcopper nitrate in 10 parts by mass of pure water was added thereto, andthis mixture was concentrated by heating with stirring until thespecific gravity of the resulting slurry reached 1.4. Thereafter, theheating was discontinued and the slurry was cooled to room temperature.The resulting slurry containing a precipitate was filtered with aNutsche type vacuum filter using No. 5C filter paper (manufactured byAdvantech Japan Co., Ltd.) to obtain 62.5 parts by mass of a wetprecipitate. This wet precipitate contained 30.15 parts by mass (12 atm.%) of molybdenum, 0.81 part by mass (1.0 atm. %) of phosphorus, 1.03parts by mass (0.30 atm. %) of cesium, 0.02 part by mass (0.015 atm. %)of vanadium, and 0.30 part by mass (0.18 atm. %) of copper; 1.2 moles ofammonium root per mole of phosphorus; and impurities such as 1.1 molesof chlorine per mole of phosphorus. When this precipitate was dispersedin the filtrate obtained in the filtration step, and used for themeasurement of its average particle diameter, it was found to be 18.1μm.

[0112] (b) Washing of the Molybdenum-Based Precipitate

[0113] The precipitate thus obtained was dispersed in 2 parts by mass ofa 2% aqueous solution of ammonium nitrate (i.e., an aqueous solutionprepared by dissolving reagent grade ammonium nitrate powder in purewater, which had a pH of 5.2 at room temperature and contained 0.25mole/L of ammonium root) and kept in the dispersed state for 5 minutes.Then, the precipitate was collected by filtration with a Nutsche typevacuum filter. This dispersion washing procedure was repeated twice.When the precipitate having been washed twice was dispersed in thefiltrate obtained in the first washing step, and used for themeasurement of its particle diameter, the average particle diameter wasfound to be 15.5 μm (with a rate of change in average particle diameterof 14.4%). The amount of chlorine contained in the precipitate was 0.10mole per mole of phosphorus.

[0114] From the above-described results, it has been found that, bywashing a molybdenum-based precipitate with an acid aqueous solutionhaving a pH of 6.5 or less and containing not less than 0.01 mole/L ofammonium root, the precipitate shows only a slight change in averageparticle diameter before and after washing, and contains only smallamounts of residual impurities.

EXAMPLE 7

[0115] A precipitate was obtained under the same conditions as employedin Example 6. This precipitate was washed twice in exactly the samemanner as in Example 6, except that a 0.5% aqueous solution of ammoniumnitrate (having a pH of 5.6 at room temperature and containing 0.06mole/L of ammonium root) was used as the washing fluid. After washing,the amount of chlorine contained in the wet precipitate was 0.10 moleper mole of phosphorus. When the precipitate was dispersed in thefiltrate obtained in the first washing step, and used for themeasurement of its average particle diameter, it was found to be 14.1 μm(with a rate of change in average particle diameter of 22.1%).

[0116] From the above-described results, it has been found that, bywashing a molybdenum-based precipitate with an acid aqueous solutionhaving a pH of 6.5 or less and containing not less than 0.01 mole/L ofammonium root, the precipitate shows only a slight change in averageparticle diameter before and after washing, and contains only smallamounts of residual impurities.

EXAMPLE 8

[0117] A precipitate was obtained under the same conditions as employedin Example 6. This precipitate was washed twice in exactly the samemanner as in Example 6, except that pure water was used as the washingfluid. After washing, the amount of chlorine contained in the wetprecipitate was 0.14 mole per mole of phosphorus, but the averageparticle diameter of the precipitate was 4.5 μm (with a rate of changein average particle diameter of 75.1%). This caused an extremedegradation in filterability and the passage of the precipitate into thefiltrate occurred.

[0118] EXAMPLE 9

[0119] (a) Preparation of a Molybdenum-Based Precipitate

[0120] 63.52 parts by mass of ammonium paramolybdate, and 7.61 parts bymass of cesium nitrate were added to 300 parts by mass of pure water,followed by the addition of 3.46 parts by mass of 85% phosphoric acid.Subsequently, 70.2 parts by mass of 25% aqueous ammonia (i.e., 34.2moles of ammonium root per mole of phosphorus) added thereto withstirring, followed by heating to 35° C. Then, 109.1 parts by mass of 36%hydrochloric acid was added dropwise thereto so as to adjust the pH ofthe mixture to 2.0. After pH adjustment, the mixture was kept for 2hours with stirring and then cooled to room temperature. The resultingslurry containing a precipitate was filtered with a Nutsche type vacuumfilter using No. 5C filter paper (manufactured by Advantech Japan Co.,Ltd.) to obtain 110.5 parts by mass of a wet precipitate. When thisprecipitate was dispersed in the filtrate obtained in the firstfiltration step, and used for the measurement of its particle diameter,the average particle diameter was found to be 11.5 μm. This wetprecipitate contained 33.91 parts by mass (12 atm. %) of molybdenum,0.93 part by mass (1.0 atm. %) of phosphorus, and 5.11 parts by mass(1.3 atm. %) of cesium; 2.40 moles of ammonium root per mole ofphosphorus; and impurities such as 0.20 mole of chlorine per mole ofphosphorus.

[0121] (b) Washing of the Molybdenum-Based Precipitate

[0122] The precipitate thus obtained was dispersed in 2 parts by mass ofa 2% aqueous solution of ammonium nitrate (having a pH of 5.2 at roomtemperature and containing 0.25 mole/L of ammonium root). After thisdispersion was stirred for 5 minutes, the precipitate was collected byfiltration with a Nutsche type vacuum filter. This washing procedure wasrepeated twice. When the precipitate having been washed was dispersed inthe filtrate obtained in the first washing step, and used for themeasurement of its average particle diameter, it was found to be 10.1 μm(with a rate of change in average particle diameter of 12.2%). Thisprecipitate contained 0.11 mole per mole of phosphorus.

[0123] From the above-described results, it has been found that, bywashing a molybdenum-based precipitate with an acid aqueous solutionhaving a pH of 6.5 or less and containing not less than 0.01 mole/L ofammonium root, the precipitate shows only a slight change in averageparticle diameter before and after washing, and contains only smallamounts of residual impurities.

EXAMPLE 10

[0124] A precipitate was obtained under the same conditions as employedin Example 9. This precipitate was washed twice in exactly the samemanner as in Example 9, except that a 1.0% aqueous solution of ammoniumnitrate (having a pH of 5.4 at room temperature and containing 0.13mole/L of ammonium root) was used as the washing fluid. When the wetprecipitate having been washed was dispersed in the filtrate obtained inthe first washing step, and used for the measurement of its averageparticle diameter, it was found to be 9.8 μm (with a rate of change inaverage particle diameter of 14.8%). The amount of chlorine contained inthe precipitate was 0.10 mole per mole of phosphorus.

[0125] From the above-described results, it has been found that, bywashing a molybdenum-based precipitate with an acid aqueous solutionhaving a pH of 6.5 or less and containing not less than 0.01 mole/L ofammonium root, the precipitate shows only a slight change in averageparticle diameter before and after washing, and contains only smallamounts of residual impurities.

EXAMPLE 11

[0126] A precipitate was obtained under the same conditions as employedin Example 9. This precipitate was washed twice in exactly the samemanner as in Example 9, except that pure water was used as the washingfluid. After washing, the amount of chlorine contained in theprecipitate was 0.18 mole per mole of phosphorus, and the averageparticle diameter of the precipitate was 2.2 μm (with a rate of changein average particle diameter of 80.9%). The times required forprecipitation after dispersion washing and for the filtration of washingfluid were about ten times longer as compared with the case in which a2% aqueous solution of ammonium nitrate was used as the washing fluid,and the passage of the precipitate into the filtrate occurred.

EXAMPLE 12

[0127] A precipitate was obtained under the same conditions as employedin Example 9. This precipitate was washed twice in exactly the samemanner as in Example 9, except that a 1% aqueous solution of nitric acidwas used as the washing fluid. After washing, the amount of chlorinecontained in the precipitate was 0.13 mole per mole of phosphorus, andthe average particle diameter of the precipitate was 4.2 μm (with a rateof change in average particle diameter of 63.5%). In this case, theprecipitate had extremely poor filterability and the passage of theprecipitate into the filtrate occurred.

EXAMPLE 13

[0128] (a) Use of a Molybdenum-Based Catalyst

[0129] A tubular reactor was charged with a catalyst containing 34.54parts by mass of molybdenum, 0.93 part by mass of phosphorus, 1.41 partsby mass of potassium, 0.76 part by mass of vanadium, and 0.57 part bymass of copper and having the composition ofP₁Mo₁₂K_(1.2)V_(0.5)Cu_(0.3), exclusive of oxygen (the same shall applyhereinafter). Then, a gaseous mixture composed of 5% by volume ofmethacrolein, 10% by volume of oxygen, 30% by volume of water vapor, and55% by volume of nitrogen was reacted for 2,000 hours under reactionconditions including a reaction temperature of 270° C. and a contacttime of 3.6 seconds.

[0130] (b) Preparation of a Molybdenum-Based Precipitate

[0131] After completion of the reaction, the catalyst was removed fromthe tubular reactor, and 91 parts by mass of this used catalyst wasdispersed in 400 parts by mass of pure water. After 89.0 parts by massof a 45% aqueous solution of sodium hydroxide was added thereto and theresulting mixture was stirred for 1 hour, the residue was filtered off.The filtrate was freed of potassium by passing it through a stronglyacid Na-type styrene-based ion-exchange resin (Amberlite IR-120B,manufactured by Organo Corp.) at a space velocity (SV) of 1. After thepH of the effluent fluid was adjusted to 9.0 by the addition of 29.0parts by mass of 36% hydrochloric acid, 19.25 parts by mass of ammoniumchloride (i.e., 12.0 moles of ammonium root per mole of phosphorus) wasadded thereto. Then, 89.2 parts by mass of 36% hydrochloric acid wasadded thereto so as to adjust the pH of the solution to 1.0, and thissolution was kept at 25° C. for 3 hours with stirring. The resultingslurry containing a precipitate was filtered with a Nutsche type vacuumfilter using No. 5C filter paper (manufactured by Advantech Japan Co.,Ltd.) to obtain 125.5 parts by mass of a wet precipitate. This wetprecipitate contained 31.91 parts by mass (12 atm. %) of molybdenum,0.91 part by mass (1.1 atm. %) of phosphorus, and 0.66 part by mass(0.47 atm. %) of vanadium; 5.52 moles of ammonium root per mole ofphosphorus; and impurities such as 1.20 moles of chlorine and 1.13 molesof sodium, per mole of phosphorus. When this precipitate was dispersedin the filtrate obtained in the first filtration step, and used for themeasurement of its particle diameter, the average particle diameter wasfound to be 9.8 μm.

[0132] (c) Washing of the Molybdenum-Based Precipitate

[0133] The precipitate thus obtained was dispersed in 2 parts by mass ofa 2% aqueous solution of ammonium nitrate (having a pH of 5.2 at roomtemperature and containing 0.25 mole/L of ammonium root). After thisdispersion was stirred for 5 minutes, the precipitate was collected byfiltration with a Nutsche type vacuum filter. This dispersion washingprocedure was repeated twice. When the precipitate having been washedwas dispersed in the filtrate obtained in the first washing step, andused for the measurement of its particle diameter, the average particlediameter was found to be 8.5 μm (with a rate of change in averageparticle diameter of 13.3%). This precipitate contained 0.10 mole ofsodium and 0.10 mole of chlorine, per mole of phosphorus.

[0134] From the above-described results, it has been found that, bywashing a molybdenum-based precipitate with an acid aqueous solutionhaving a pH of 6.5 or less and containing not less than 0.01 mole/L ofammonium root, the precipitate shows only a slight change in averageparticle diameter before and after washing, and contains only smallamounts of residual impurities.

[0135] Moreover, it has been found that a molybdenum-based precipitatecan also be satisfactorily washed when the potassium contained in theabove molybdenum-based precipitate is replaced by sodium.

EXAMPLE 14

[0136] A precipitate was obtained under the same conditions as employedin Example 13. This precipitate was washed in the same manner as inExample 13, except that the 2% aqueous solution of ammonium nitrate wasreplaced by a 2% aqueous solution of ammonium chloride (having a pH of5.3 at room temperature and containing 0.37 mole/L of ammonium root).When the wet precipitate having been washed was dispersed in thefiltrate obtained in the first filtration step, and used for themeasurement of its average particle diameter, it was found to be 8.5 μm(with a rate of change in average particle diameter of 13.3%). Theamount of sodium contained in this precipitate was 0.09 mole per mole ofphosphorus.

[0137] From the above-described results, it has been found that sodiumcan be reduced by using an aqueous solution of ammonium chloride.

EXAMPLE 15

[0138] A precipitate was obtained under the same conditions as employedin Example 13. This precipitate was washed twice in exactly the samemanner as in Example 13, except that pure water was used as the washingfluid. When the wet precipitate having been washed was dispersed in thefiltrate obtained in the first filtration step, and used for themeasurement of its average particle diameter, it was found to be 3.1 μm(with a rate of change in average particle diameter of 68.4%). Thisprecipitate contained 0.12 mole of sodium and 0.16 mole of chlorine, permole of phosphorus. After washing, the precipitate had extremely poorfilterability and the passage of the precipitate into the filtrateoccurred.

EXAMPLE 16

[0139] (a) Preparation of a Molybdenum-Based Precipitate

[0140] A catalyst containing 34.54 parts by mass of molybdenum, 0.93part by mass of phosphorus, 5.18 parts by mass of cesium, and 1.12 partsby mass of arsenic and having the composition of P₁Mo₁₂As_(0.5)Cs_(1.3)was used for 2,000 hours under the same reaction conditions as inExample 13. 89 parts by mass of this used catalyst was dispersed in 400parts by mass of pure water. After 89.0 parts by mass of a 45% aqueoussolution of sodium hydroxide was added thereto and the resulting mixturewas stirred for 3 hours, a small amount of undissolved matter wasfiltered off to obtain a homogeneous solution. After the pH of thesolution was adjusted to 7.5 by the addition of 33.5 parts by mass of36% hydrochloric acid, 28.90 parts by mass of ammonium chloride (i.e.,12.0 moles of ammonium root per mole of the combined amount ofphosphorus and arsenic) was added thereto. Then, 56.7 parts by mass of36% hydrochloric acid was added thereto so as to adjust the pH of thesolution to 4.0, and this solution was kept at 60° C. for 3 hours withstirring. The resulting slurry containing a precipitate was filteredwith a Nutsche type vacuum filter using No. 5C filter paper(manufactured by Advantech Japan Co., Ltd.) to obtain 78.5 parts by massof a wet precipitate. This wet precipitate contained 26.95 parts by mass(12 atm. %) of molybdenum, 0.51 part by mass (0.71 atm. %) ofphosphorus, 1.12 parts by mass (0.64 atm. %) of phosphorus, and 5.10parts by mass (1.6 atm. %) of cesium; 9.52 moles of ammonium root permole of phosphorus; and impurities such as 0.93 mole of chlorine and0.76 mole of sodium, per mole of the combined amount of phosphorus andarsenic. When this precipitate was dispersed in the filtrate obtained inthe first filtration step, and used for the measurement of its particlediameter, the average particle diameter was found to be 18.5 μm.

[0141] (b) Washing of the Molybdenum-Based Precipitate

[0142] The precipitate thus obtained was dispersed in 2 parts by mass ofa 2% aqueous solution of ammonium nitrate (having a pH of 5.2 at roomtemperature and containing 0.25 mole/L of ammonium root). After thisdispersion was stirred for 5 minutes, the precipitate was collected byfiltration with a Nutsche type vacuum filter. This washing procedure wasrepeated twice. When the precipitate having been washed was dispersed inthe filtrate obtained in the first washing step, and used for themeasurement of its particle diameter, the average particle diameter wasfound to be 16.8 μm (with a rate of change in average particle diameterof 9.2%). This precipitate contained 0.06 mole of sodium and 0.05 moleof chlorine, per mole of the combined amount of phosphorus and arsenic.

[0143] From the above-described results, it has been found that, bywashing a molybdenum-based precipitate with an acid aqueous solutionhaving a pH of 6.5 or less and containing not less than 0.01 mole/L ofammonium root, the precipitate shows only a slight change in averageparticle diameter before and after washing, and contains only smallamounts of residual impurities.

EXAMPLE 17

[0144] A precipitate was obtained under the same conditions as employedin Example 16. This precipitate was washed twice in the same manner asin Example 16, except that a 2% aqueous solution of ammonium sulfate(having a pH of 6.0 at room temperature and containing 0.30 mole/L ofammonium root) was used as the washing fluid. When the wet precipitatehaving been washed was dispersed in the filtrate obtained in the firstfiltration step, and used for the measurement of its average particlediameter, it was found to be 15.5 μm (with a rate of change in averageparticle diameter of 16.2%). This precipitate contained 0.07 mole ofsodium and 0.05 mole of chlorine, per mole of the combined amount ofphosphorus and arsenic.

EXAMPLE 18

[0145] A precipitate was obtained under the same conditions as employedin Example 16. This precipitate was washed twice in the same manner asin Example 16, except that pure water was used as the washing fluid. Theaverage particle diameter of the precipitate was found to be 4.6 μm(with a rate of change in average particle diameter of 75.1%). Afterwashing, the precipitate had extremely poor filterability and thepassage of the precipitate into the filtrate occurred.

Exploitability in Industry

[0146] It is evident from the above description that, by employing apreparation process including at least the first step of forming a crudeprecipitate by pH adjustment to 6.5 or less in the presence of an alkalimetal compound and the second step of dissolving the crude precipitatein aqueous ammonia and forming a precipitate by pH adjustment to 6.5 orless, a molybdenum-based precipitate having a high purity and a desiredaverage particle diameter can be conveniently prepared with goodworkability, even when the molybdenum-based precipitate contains threeor more metallic elements.

[0147] Moreover, by washing a molybdenum-based precipitate with an acidaqueous solution having a pH of 6.5 or less and containing not less than0.01 mole/L of ammonium root, a change in the average particle diameterof the molybdenum-based precipitate can be suppressed and, moreover, themolybdenum-based precipitate can be conveniently washed with goodworkability. Thus, there can be obtained a molybdenum-based precipitatehaving a high purity and a desired average particle diameter.Furthermore, a molybdenum-based precipitate containing three or moremetallic elements can also be washed satisfactorily.

What is claimed is:
 1. A process for the preparation of a precipitate containing at least molybdenum and an element A (in which A is at least one element of phosphorus and arsenic), the process including: the first step of adjusting the pH of a fluid mixture containing the constituents of the precipitate to 6.5 or less in the presence of an alkali metal compound, and filtering the crude precipitate so formed; and the second step of dissolving the crude precipitate in aqueous ammonia, adjusting the pH of the resulting solution to 6.5 or less, and filtering the precipitate so formed.
 2. A process for the preparation of a precipitate as claimed in claim 1 wherein, subsequently to at least one of said first step and said second step, at least one of the crude precipitate obtained in said first step and the precipitate obtained in said second step is washed with an acid aqueous solution having a pH of 6.5 or less and containing not less than 0.01 mole/L of ammonium root.
 3. A process for the preparation of a precipitate as claimed in claim 2 wherein the amount of chlorine contained in the washed precipitate obtained by washing the precipitate obtained in said second step is not greater than 0.10 mole per mole of the element A.
 4. A process for the preparation of a precipitate as claimed in claim 2 wherein the amount of sodium contained in the washed precipitate obtained by washing the precipitate obtained in said second step is not greater than 0.10 mole per mole of the element A.
 5. A process for the preparation of a precipitate as claimed in claim 1 wherein, when the constituents of the precipitate include at least one element selected from the group consisting of potassium, rubidium and cesium, the at least one element selected from the group consisting of potassium, rubidium and cesium is substantially removed prior to said first step.
 6. A process for the preparation of a precipitate as claimed in claim 1 wherein the fluid mixture containing the constituents of the precipitate is prepared by providing a catalyst containing at least molybdenum, the element A and an element Z (in which Z is at least one element selected from the group consisting of potassium, rubidium and cesium) and having been used for the purpose of a reaction, dispersing the catalyst in water, and adding at least one of an alkali metal compound and aqueous ammonia to the dispersion.
 7. A process for the preparation of a precipitate as claimed in claim 6 wherein the catalyst containing at least molybdenum, the element A and the element Z and having been used for the purpose of a reaction is a catalyst for the production of methacrylic acid by the vapor phase catalytic oxidation of methacrolein.
 8. A process for the preparation of a precipitate which includes the step of washing a precipitate containing at least molybdenum and an element A (in which A is at least one element of phosphorus and arsenic) with an acid aqueous solution, wherein the acid aqueous solution has a pH of 6.5 or less and contains not less than 0.01 mole/L of ammonium root.
 9. A process for the preparation of a precipitate as claimed in claim 8 wherein the precipitate is derived from a catalyst containing at least molybdenum, the element A and an element Z (in which Z is at least one element selected from the group consisting of potassium, rubidium and cesium) and having been used for the purpose of a reaction.
 10. A process for the preparation of a precipitate as claimed in claim 9 wherein the catalyst containing at least molybdenum, the element A and the element Z and having been used for the purpose of a reaction is a catalyst for the production of methacrylic acid by the vapor phase catalytic oxidation of methacrolein.
 11. A process for the preparation of a molybdenum-based catalyst represented by the following general formula (1): A_(a)Mo_(b)V_(c)Cu_(d)D_(e)Y_(f)Z_(g)O_(h)   (1) wherein A is at least one element of phosphorus and arsenic, D is at least one element selected from the group consisting of antimony, bismuth, germanium, zirconium, tellurium, silver, selenium, silicon, tungsten and boron, Y is at least one element selected from the group consisting of iron, zinc, chromium, magnesium, tantalum, manganese, cobalt, barium, gallium, cerium and lanthanum, Z is at least one element selected from the group consisting of potassium, rubidium and cesium, a is in the range of 0.5 to 3, b is equal to 12, c is in the range of 0 to 3, d is in the range of 0 to 3, e is in the range of 0 to 3, f is in the range of 0 to 3, and h is the atomic ratio of oxygen required to meet the valence of each element, the process including: the first step of adjusting the pH of a fluid mixture containing at least molybdenum and the element A to 6.5 or less in the presence of an alkali metal compound, and filtering the crude precipitate so formed; the second step of dissolving the crude precipitate in aqueous ammonia, adjusting the pH of the resulting solution to 6.5 or less, and filtering the precipitate so formed; and the step of preparing the catalyst from the precipitate.
 12. A process for the preparation of a catalyst as claimed in claim 11 wherein, subsequently to at least one of said first step and said second step, at least one of the crude precipitate obtained in said first step and the precipitate obtained in said second step is washed with an acid aqueous solution having a pH of 6.5 or less and containing not less than 0.01 mole/L of ammonium root.
 13. A process for the preparation of a catalyst as claimed in claim 11 wherein the catalyst is a catalyst for the production of methacrylic acid by the vapor phase catalytic oxidation of methacrolein. 